An emulsion is formed when two immiscible liquids, usually water or a water-based solution and a hydrophobic organic liquid (e.g., an oil) are mixed so that one liquid forms droplets in the other liquid. Either of the liquids can be dispersed in the other liquid. When, for example, oil is dispersed in water, the emulsion is referred to as an oil-in-water (o/w) emulsion. The reverse case is a water-in-oil (w/o) emulsion.
If the emulsion is not stabilized by using certain methods, typically by adding surfactants or emulsifiers, the emulsion tends to agglomerate, form a creaming layer, coalesce, and finally separate into two phases. If a surfactant or emulsifier (sometimes referred to as a surface-active agent) is added to one or both of the immiscible liquids, one of the liquids forms a continuous phase and the other liquid remains in droplet form (“disperse or dispersed phase”), the droplets being dispersed in the continuous phase. On the other hand, the degree of stability without a surfactant is increased when droplet size is decreased below certain values. For example, a typical o/w emulsion of a droplet size of 20 microns may be only temporally stable (hours) while that of 1 micron may be considered as “quasi-permanently” stable (weeks or longer). However, the energy consumption and the power requirement for the emulsification system and process are drastically increased for smaller droplet size in conventional mixing and agitation hardware, especially for highly viscous emulsions of very small droplet size and large output. It has been observed that the doubling of energy dissipation (energy consumption) only causes a reduction of average droplet size of 25%. Shear force is usually applied to overcome the surface tension force of the dispersed phase and in turn to breakup larger droplets into smaller ones. It is known that as the size of the droplet size decreases, the surface tension required to keep the droplet shape increases. Energy consumption takes place in various forms, for example, it can be the energy needed by the stirrer to overcome shear force of the emulsion in a batch process, the energy for heating and cooling, and/or the power to overcome pressure drop in a continuous process. Heating is often needed for emulsification when one of the phases does not flow at room temperature. A heated emulsion has lower stability, however, due to lower viscosity of the continuous phase and in turn less drag. Drag is necessary to stop the motion of the droplets and in turn the coalescence. After emulsification, droplets tend to rise by buoyancy. As such, an immediate cooling down is usually needed, which also consumes energy.
The type of emulsion (o/w or w/o) may be determined by the volume ratio of the two liquids provided the ratio is relatively high. For example, with 5% water and 95% oil (an w/o phase ratio of 0.0526), the emulsion will typically become w/o unless measures are taken to provide for the formation of an o/w emulsion. However, for a uniform droplet size of volumetrical ratio between 0.26% and 74% both o/w and w/o emulsions are possible. For non-uniform droplet size distribution this range of ratio could be larger.
For moderate phase ratios (>0.333), the type of emulsion may be decided by several factors, such as order of addition or type of surfactant. One liquid slowly added to the other with agitation usually results in the last-mentioned liquid being the continuous phase. Another factor is preferred solubility of the surfactant; the phase in which the surfactant is soluble typically is continuous.
Occasionally, inversion takes place; an o/w emulsion changes into w/o emulsion or vice versa. More complex emulsions such as double emulsions may be formed when the water droplets in a continuous oil phase themselves contain dispersed oil droplets. Such oil-in-water-in-oil emulsions are noted as o/w/o. In the same manner a w/o/w emulsion may be formed.
A problem with many of the emulsions that are currently available relates to the presence of surfactants or emulsifiers in their formulations, or at least relatively high concentrations of such surfactants or emulsifiers. These surfactants or emulsifiers are required in order to stabilize the emulsions, but are undesirable for many applications. For example, heating without bubbling or boiling is often desired in emulsification processes, however it is found that the onset temperature of nucleate boiling or air bubble formation from dissolved air in the continuous phase lowers when surfactants or emulsifiers are present. Boiling may cause unwanted property changes. Air bubbles cause more creaming and other undesired features. Another example relates to the fact that low-surfactant or surfactant free emulsions are highly desirable for skin care products in the cosmetic industry. A major disadvantage of some surfactants or emulsifiers is their tendency to interact with preservatives such as the esters of p-hydroxybenzoic acid. Skin irritation is another problem often associated with the use of surfactants or emulsifiers. Many adverse skin reactions experienced by consumers from the use of cosmetics are directly related to the presence of the surfactants or emulsifiers. Another example relates to the problem with using surfactants wherein water proofing is desired. For example, in water-based paints, as well as certain skin care products such as sunscreen, the active ingredient is not waterproof due to the presence of water-soluble surfactants. The inventive process provides a solution to this problem.
A problem relating to the use of many pharmaceutically acceptable compounds or drugs relates to the fact that they are insoluble or poorly soluble in water and there are limitations as to the surfactants or emulsifiers that may be used. This has resulted in the discovery of drugs that are not clinically acceptable due to problems relating to transporting the drugs into the body. Emulsion formulation problems are particularly problematic with drugs for intravenous injection and the administration of chemotherapeutic or anti-cancer agents. This invention provides a solution to this problem.
The inventive process involves making emulsions using microchannel process technology. In one embodiment, the process requires relatively low energy consumption and provides for low continuous phase pressure drop and low pressure drop between the bulk dispersion phase and the continuous phase. The emulsions made using this process, at least in one embodiment, are characterized by a dispersed phase with a relatively small and relatively uniform droplet size. These emulsions exhibit a high degree of stability as a result of the small droplet size and the controlled droplet size distribution in the dispersed phase. This creates the potential for making low-surfactant or surfactant-free emulsions which are desirable for many applications, including skin care products in the cosmetic industry.